The present invention relates generally to self-cleaning coatings and, more particularly, to the adhesion of photocatalytic dielectric elements placed upon polytetrafluoroethylene (PTFE). The invention reflects UV light while concurrently inducing photocatalytic reactions of anatase TiO2 on the surface of an optical substrate (PTFE), and is used for producing self-cleaning refractors and lamp covers on roadway lighting fixtures.
Outdoor lighting fixtures are often exposed to harsh conditions, such as weather, exhaust from vehicles, air pollution from factories, dirt and grime kicked up from the road, etc. All these factors and more contribute to reduce light output from roadway lighting fixtures. Therefore, the refractors or lamp covers on the fixtures require continuous cleaning. However, if the degradation is too bad, the refractor or lamp cover must be replaced. Most refractors and lamp covers are made of plastic, but exposure to UV light, such as the UV light present in sunlight, can cause a variety of problems for these plastic covers. For example, UV light can cause plastic materials, such as acrylic and polycarbonate materials, to craze as a result of photo-induced chemical cross-linking. Plastics exposed to UV light develop a network of fine cracks, yellowing, and loss of physical properties.
Polytetrafluoroethylene (PTFE) is appealing because it has excellent impact resistance and weatherability. PTFE also has excellent xe2x80x9cplastic memory,xe2x80x9d meaning that if compressive or stress forces are applied to the PTFE, it has the ability to retain its original form or shape. Further, PTFE is not affected by UV and has excellent transmissive properties. Though the surface is considered xe2x80x9cnon-stickxe2x80x9d and slippery, static build-up can cause matter to be attracted to the surface and result in some loss in light transmission.
Methods have been investigated in using photocatalysts on the surface of light fixtures to provide a self-cleaning surface. Currently, photocatalytic films are deposited on various substrate materials using the Sol-Gel method. The film comprises one or more layers of photoreactive gelatin which have been subsequently developed by wet chemical processing. Such a process is disclosed in xe2x80x9cApplications of Photocatalytic Reactions Caused by TY02 Film to Improve the Maintenance Factor of Lighting Systems,xe2x80x9d by H. Honda, A. Ishizala (1), R Soma (2), K. Hashimoto, and A. Fujishima (3), Winter 1998; Journal of the Illuminating Engineering Society. Taught is a substrate which is dipped into a titanium alkoxide solution, TPT monomer or polymer chelated with glycol polymer. There may be variations in the mixture as far as what is used, but the process manner is the same such that the rate in which the substrate is pulled out determines the coating thickness. The coated substrate is then heated to about 600xc2x0 C. to form the crystalline anatase phase. However, the sol-gel process is very limited in that very few substrate materials can be exposed to such high temperatures without deforming, burning, out-gassing or melting.
In addition, it is usually desirable to construct the film with fringes that are parallel to the surface of the gelatin. However, design constraints, such as optically recorded noise patterns, may prevent this construction. In these cases, the fringes intersect the surface and form a slant fringe pattern, which produces extraneous diffraction images. Such diffraction images can make this process unacceptable for roadway lighting fixtures.
Another limitation of the sol-gel method is mechanical abrasive damage to which the film is extremely susceptible. Still a further problem of the sol-gel method is the tendency of the layers of the film to delaminate both from the substrate and from other adjacent layers therein. The delamination is due to the differing coefficients of expansion between the various layers of the film, such that as the substrate is subjected to varying thermal conditions, the layers expand and shrink at varying ratios resulting in delaminations between the layers and substrate.
It would be desirable to develop a photocatalytic anatase TiO2 film that is hard, durable and abrasion resistant, which will not delaminate or distort surfaces. It is also desirable that such a film could be deposited on polytetrafluoroethylene and on a variety of other substrate materials, such as plastics, metals, glass, and composites to be used in various applications, such as lighting, window, and other optical applications, to provide a self-cleaning surface.
The self-cleaning roadway lighting fixture herein disclosed comprises a polytetrafluoroethylene (PTFE) refractor, or lamp cover, upon which a number of alternating layers of thin dielectric films are deposited to form the photocatalytic dielectric element of the present invention. The PTFE can be transmissive for wavelengths of light or non-transmissive to wavelengths of light. In both cases the photocatalytic dielectric element deposited on the PTFE substrate is highly reflective for wavelengths of light within a predetermined spectrum and is otherwise transmissive. The photocatalytic dielectric element is an optically clear, multilayered, hard, durable, thin film comprised of an external contact layer of photocatalytic anatase TiO2, reflects greater than 90% of UV light having a wavelength of 350 nm to 400 nm, and is comprised of a series of tailored thin film dielectric reflectors designed with narrow contoured spectral bandwidths to reflect light within a predetermined spectrum. The photocatalytic dielectric element filters light and reflects selected wavelengths of light from the lamp within the light fixture or from direct sun light. The selected wavelength of light is then reflected back to the external contact layers of the photocatalytic dielectric element producing a concentration of photons at the external surface of the element, thus initiating photocatalytic reactions at the surface of the element. This creates a self-cleaning surface on the PTFE substrate that decomposes most bacteria and organic contaminates that contact the surface.
The photocatalytic dielectric element eliminates the occurrence of distortion which can possibly occur with the sol-gel method. Additionally, the photocatalytic dielectric element is less likely to delaminate from the PTFE and does not require environmental protection. The photocatalytic dielectric element is less susceptible to mechanical abrasion damage and does not require complex wet chemical development, as does the sol-gel method. The photocatalytic dielectric element is functional in both indoor and outdoor applications. The element can be placed on almost any substrate material, including, but not limited to, polytetrafluoroethylene, plastics, metals, polymers, or composites to achieve high UV reflectance and high photopic transmittance. It also provides surfaces the photocatalytic dielectric element is placed upon with a self-cleaning surface. The photocatalytic dielectric element has been preferably deposited upon a polytetrafluoroethylene substrate using dual alternate reduced pressure environment techniques.
Thin, optical film, computer design codes with optimization routines are commercially available with which dielectric coatings can be tailored. A suitable software package is FILMSHOW by FTG Software Associates, Princeton, N.J.
It has been generally accepted that the adhesion between a thin dielectric film and a soft, nonstick material, such as PTFE, would be too weak, and not capable of providing any lasting adhesive bonds. The arguments would be that any amount of flex in the material causes the thin dielectric film to fracture and delaminate from the PTFE, or that the fracturing would ruin the function of the dielectric element. An aspect of this invention is that alternating layers of thin dielectric films with good adhesion to PTFE are possible. Furthermore, if the photocatalytic dielectric element should contain cracks or fractures, it would still function adequately as a self-cleaning element. In addition, if during its lifetime the dielectric layers become fractured, its function as a photocatalytic dielectric element continues, with some loss regarding transmission of light. However, the overall performance of the photocatalytic dielectric element exceeds that of organic methods, such as the sol-gel method.